1. Field of the Invention
The present invention is generally related to plasma processing and, more particularly, to a method and apparatus for determining plasma impedance using a modeled radio frequency transmission line.
2. Discussion of the Background
Plasmas are widely used in a variety of integrated circuit fabrication processes, including plasma etching and plasma deposition applications. Generally, plasmas are produced within a processing chamber by introducing a low-pressure process gas into the chamber and then directing electrical energy into the chamber for creating an electric field therein. The electric field creates an electron flow within the chamber that ionizes individual gas molecules by transferring kinetic energy to the molecules through individual electron-gas molecule collisions. The electrons are accelerated within the electric field, producing efficient ionization of the gas molecules, and the ionized particles of the gas and the free electrons collectively form what is referred to as plasma.
At all stages of plasma processing, different measurements and assessments regarding processing components and the plasma itself are required. For example, the determination of the state of the plasma at any given time is often desired. Determining plasma state in a chamber requires the experimental determination of plasma properties, which can be estimated using a measured plasma radio frequency (“RF”) impedance, which can be found at a number of frequencies (e.g., full plasma impedance spectra). However, plasma is an inherently difficult medium to handle, and direct measurement of plasma RF impedance in the chamber is nearly impossible, as it is very difficult to precisely install a voltage-current (“VI”) probe at a boundary between the plasma and a chamber wall. Specifically, in the case of a capacitive-coupled system, there would typically be an electrode driving the plasma, surrounded by an insulator. The RF electric potential is applied across the insulator, between the electrode and the plasma chamber wall. The insulator typically serves multiple purposes, including being vacuum tight. The ideal location for a voltage and current probe would be exactly on the interface between the plasma and the insulator, such that the current is measured passing along the electrode wall, at that plasma-insulator interface; and voltage is measured across the insulator at that same location. Only in this fashion, with the voltage and current probes directly adjacent the plasma can one directly measure the impedance that the plasma presents to the RF drive circuit. Voltage and current probes have finite dimensions (e.g. a few centimeters large, at least) and those prevent them from being installed at the very plasma-insulator interface.
Moreover, even if the probes can be made very small, and therefore, possible to install sufficiently near the interface, there still remain the mechanical and electrical problems of leading the RF signals out via wires, etc., through the insulator which needs to be vacuum tight. Furthermore, wideband RF signals typically need some sort of signal conditioning done at the place of collection to be able to be transmitted via transmission lines without deterioration, and these signal conditioning circuits need to be installed inside the insulator as well. In all, it would be advantageous to install the voltage and current probes at a more suitable location along the RF drive transmission line, and extract the plasma impedance from the impedance measurement at this location.
The interior condition of the chamber itself is further information that needs to be assessed, as deposition attributable to the plasma adheres to inner surfaces of the chamber during plasma processing. When such deposition accumulates to a certain amount, it separates from the inner surfaces to become loose particles that can contaminate a wafer being processed. However, the interior condition of a chamber is not readily apparent to a system operator, such that frequent disassembly and cleaning must be performed to ensure that the chamber is in a proper condition for plasma processing. Because of this, a chamber may be dissembled and cleaned even when the amount of deposition has not accumulated to an amount detrimental to plasma processing, resulting in unnecessary downtime of the processing tool. Moreover, when the chamber is disassembled and reassembled for preventive maintenance, no current method exists for ascertaining that the chamber exhibits the same RF characteristics as before disassembly and/or being contaminated by process.
A number of techniques have been developed to address the above-described concerns; however, such techniques have proven to be intrusive to the plasma process